The measurement of various substances by the use of radioimmunoassay techniques has achieved widespread acceptance in recent years. Yalow and Berson, In Vitro Procedures With Radioisotopes In Medicine, International Atomic Energy Agency, Vienna (1970) pgs. 455 et seq., express the principle of radioimmunoassay in the following terms:
"Unlabelled antigen in unknown samples competes against labelled antigen ("tracer") for binding to antibody and thereby diminishes the binding of labelled antigen. The degree of competitive inhibition observed in unknown samples is compared with that obtained in known standard solutions for determination of concentration of antigen in unknowns."
Radioimmunoassay tests require a specific antibody, a radioisotope-labeled (hereinafter referred to as "radio-labeled") antigen, a pure sample of the antigen to be measured to serve as a reference standard, and means for the separation of free antigen from antibody-bound antigen. Radioimmunoassays follow the basic principle of saturation analysis, i.e., competition between labeled and unlabeled antigen for a fixed number of antibody binding sites.
When radiolabeled antigen, unlabeled antigen, and antibody are brought together, the amount of radiolabeled antigen bound to antibody and the amount of radiolabeled antigen remaining unbound (free) has a direct relationship to the amount of unlabeled antigen present when a given amount of antibody is present. Thus, by using a constant amount of antibody and radiolabeled antigen, and using known concentrations of unlabeled antigen, a standard (calibration) curve can be plotted showing antigen concentration versus the amount of radiolabeled antigen bound or versus radiolabeled antigen unbound, or versus a ratio of the two measurements. The concentration of antigen in an unknown sample can be read from the standard curve by determining the amount of bound or free radiolabeled antigen (or ratio of the two measurements) resulting when the unknown sample is mixed with the amount of radiolabeled antigen and antibody used to prepare the curve. In all radioimmunoassay procedures it is necessary to provide means for separating the bound from the free labeled tracer material. Many widely varied procedures have been developed and used; exemplary procedures are electrophoresis; chromatography; ion exchange; adsorption to dextran coated charcoal, talc, or cellulose; and a number of solid-phase antibody techniques.
The term "antigen," as used in the field of radioimmunoassays, may cover substances of limited immunogenicity (ability to generate antibodies). In those cases where the substance to be measured is of limited immunogenicity, the substance can be coupled with an immunogenic carrier, usually a protein, to increase its immunogenicity. A substance that is nonimmunogenic, but acquires immunogenicity when linked with a carrier is referred to as a "hapten."
Radioimmunoassay techniques have been used to determine the concentration in body fluids of various endogenous and exogenous steroids. In the development of radioimmunoassays for the various steroids, the preparation of a radiolabeled antigen is of primary concern. Possible radioisotope labels are tritium, carbon-14, iodine-125, iodine-131, and others. However, because tritium and carbon-14 must be counted by liquid scintillation (a time-consuming and expensive process), iodine-125 and iodine-131 are more desirable. For reasons well-recognized in the art (e.g., half-life, radiation hazard, counting efficiency and others) iodine-125 has become the radioisotope of choice for use in steroid radioimmunoassays.
The chemical structure of steroids is such that it is generally not possible to radioiodinate them directly. It is necessary, therefore, to utilize as a precursor of the radiolabeled antigen a derivative of the steroid to be assayed which is readily iodinated. In choosing or developing such a derivative, the primary concern is the affinity of the derivative for the antibodies of the steroid to be assayed; the affinity of the derivative for the antibodies should, of course, be as close to the affinity of the steroid for the antibodies as possible.